This invention generally relates to traveling cranes. More particularly, this invention relates to systems for reducing friction during movement of a traveling crane along rails.
Portal cranes are used extensively in ports to load and unload ships and submarines. These cranes generally have a high load lifting capacity and therefore utilize double flange steel wheel trucks on heavy weight steel rails. The rails have a wide gage (up to 40 ft. or more). Depending on the load lifting capacity, portal cranes have a large number (8 to 16 or more) of two wheel trucks. One-half to one-third of the wheels are powered. Drive motors are generally located on the truck.
FIGS. 1A and 1B show a typical diagram of a modern portal crane 1 used by the U.S. Navy. A superstructure of several levels, which levels include a rotating upper works 10 connected to a system of booms, pulleys, hook hoists, steel ropes, etc. collectively designated at 15, which enables the crane 1 to pick up or lower heavy loads. The particular components of the lifting system or boom 15 will vary according to the intended use of the crane 1. The upper works 10 and boom 15 are supported by a traveling portal base 16, which is built on top of a plurality of trucks 11, 12, 13, 14. The trucks 11, 12, 13, 14 move on heavy rails 37, 38 placed with a wide gage 7. Similar cranes with smaller booms and smaller load lifting capacities are used by commercial services. There are several differences between Navy and commercial cranes but the type of trucks and wheels used in both cranes, collectively referred to herein as traveling cranes, is the same.
In Navy portal cranes, power is supplied by an on-board engine and generator typically located in the traveling portal base 16, above the trucks. Many of the other electrical and mechanical systems are located in a chamber-like structural member 28 of the crane 1. An on-board fuel tank supplies fuel for the engine. The maximum power available is thus limited to the capacity of the engine-generator combination. This power is used for several functions of the crane, including: moving the crane by powering the motors and driving the trucks; rotating the upper works to which the boom is connected; picking up and lowering the load; and changing the height of the boom.
A major fraction of the total power of both Navy and commercial cranes is used in moving the crane. Portal cranes travel on heavy gage rail track 37, 38 which is both tangent and curved in the shipyard. The track has a very wide gage 7 (12 ft to 44 ft and more) and has very sharp curves around the bay in the dock. Commercial cranes typically travel on straight or gently curved tracks. The peak power required to move the cranes depends on the sharpness of the curve. Even on tangent track, portal cranes use much more power with considerable noise and vibration than they need to.
Typical portal cranes have a large number of two wheel trucks, which operate on sharp curves. This requires some trucks to move laterally by several feet when they are on the curves. This also involves a sharp change of rolling direction of the wheels which are operating on curves. Each truck is free to rotate about its vertical axis, but the rolling direction of the truck wheels is not aligned perfectly when entering a curving rail. As illustrated in FIG. 2, the rolling direction 8 of the wheel 17 is different from the direction 9 of the curved rail 18. The angle 19 between them defines the lateral angle of slip or creep between the wheel 17 and the rail 18. The wheel 17 must slip laterally by a certain distance every moment in order to stay on the rail 18. This slip is given by the distance 20.
The greater the angle 19, the larger is the slip 20 and a corresponding lateral friction force, generally designated in FIG. 3 at 21. Hence, sharp turns result in the greatest lateral friction force. This lateral friction force 21 is opposed by an equal and opposite force 22 with which one flange 23 of the wheel 17 presses against the rail 18. It is generally not understood that an angle 19 as small as half a degree can produce lateral forces per wheel of several thousand pounds.
This force causes significant rail and wheel flange wear and can cause the flange 23 to break in extreme cases. In addition to creating an unsafe condition, replacing a wheel on one of these cranes is an expensive process. In other cases the flange 23 can climb on the rail 18, resulting in a derailment. Another problem associated with this process is the production of very high levels of noise, which compromises the safety of the workers underneath the crane because of their inability to talk to each other while the crane is moving. Other problems include excessive vibration and shock to both the electrical and mechanical drive trains and to the whole crane.
Yet another problem is that a major part of the energy of the power plant of the crane is used up in overcoming the wheel-rail contact friction in the lateral direction. At times, such a large part of the generator current is used to overcome this friction, that only one operating function of the crane can be performed at a time, otherwise the electrical system trips and blowouts can occur. For example, crane movement cannot occur simultaneously with the rotation of the upper works 10 or lifting of the load, so the capability of rotating the upper works while traveling around the curve (preferred by the operators) is compromised. Similarly, if the current draw by the truck motors is excessive, the electrical system trips and work is halted until it is fixed. This can happen in the middle of a load lift, leaving the load hanging in the air. Hence, any breakdown of the crane significantly reduces productivity and safety and should be avoided.
The above problems are only aggravated by the tendency of the wheels to stick as they slip along the rail which, when combined with the associated large lateral friction, causes the whole crane to vibrate and move jerkily. Nothing can be done about the distances slipped because they are defined by the geometry of the wheel and the rail. Therefore, the only way to reduce the detrimental wastage of crane energy is to reduce the friction force 21 between the wheels 17 and the rail 18.
Accordingly, a general object and aspect of the present invention is to provide a system whereby the above friction-related problems of prior art traveling cranes are substantially reduced or eliminated.
Other aspects, objects and advantages of the present invention, including the various features used in various combinations, will be understood from the following description according to preferred embodiments of the present invention, taken in conjunction with the drawings in which certain specific features are shown.